Among energy storage devices, some particularly used micro-batteries, known as “all-solid”, are in the form of films: all of the components of the micro-battery, in other words the current collectors, the positive and negative electrodes, the electrolyte, and even the encapsulation, are thin films, obtained by deposition, principally by physical vapour deposition (PVD) or chemical vapour deposition (CVD). The techniques used enable objects of any shape to be formed.
As usual, the operating principle of such a battery is based on insertion and removal of an ion of alkali metal or a proton in and from the positive electrode, and deposition or extraction of said ion on and from the negative electrode; the principal systems use Li+ as current carrying ion species. The recharging of a micro-battery, in other words the transfer of ions from the anode to the cathode, is in general complete after several minutes of charging.
Most current micro-batteries are formed in planar shape. Depending on the materials used, the operating voltage of this type of battery is between 1 and 4 V; the value of the operating voltage is only limiting for certain applications, such as sensors or actuators, which require voltages higher than several tens of volts.
The conventional surface capacity of an all-solid micro-battery is of the order of several 10 μAh/cm2 to several hundreds of μAh/cm2, which is low and remains a limiting factor for its use.
In order to increase the capacity, it is possible to increase the thicknesses of the electrode layers. However, the higher resistance linked to the diffusion of lithium throughout the material leads to poorer power performance.
Another solution proposed to increase the capacity without losing the power performance is the use of a larger developed surface. The document U.S. Pat. No. 6,495,283 thus describes a micro-battery in which one of the underlying layers, namely the substrate, the collector or the cathode, has a three-dimensional structure comprising cavities: the successive subsequent layers are deposited on the three dimensional structure, and thereby form layers of larger developed surface.
Nevertheless, with such a system, it is difficult to reach a high height to width ratio of the cavities: PVD techniques, in particular necessary for the electrolyte, do not enable a regular and homogeneous coating of the lateral (vertical) and base walls for cavities too deep and narrow. Moreover, the entire volume serving to define the three-dimensional structure is constituted by the material of the substrate, which is electrochemically inactive: the energy volume density is low.